Second-harmonic generation with the GaAs laser

The second harmonic of pulsed GaAs injection lasers was generated with the nonlinear crystal α-iodic acid (HIO
3
) phase matched by angular tuning. The injection laser was operated with an external cavity, and the HIO
3
crystal was placed inside the cavity to increase the conversion efficiency. The maximum harmonic power observed was 15 μW with a fundamental power of 4.5 watts inside the cavity. Since the lasing bandwidth was typically 30 Å and the calculated bandwidth for collinear harmonic generation was only 6.3 Å, a grating was used to narrow the lasing bandwidth to 3 Å. The expected large increase in harmonic power did not occur. It is shown that this is due to sum-frequency generation by laser modes lying outside the harmonic bandwidth. It is also found that the spectral width of the harmonic becomes bigger than the collinear bandwidth when a divergent laser beam is used.